An optical fiber grating (a fiber grating, hereinafter) has a property of reflecting optical signal with a particular wavelength propagating therethrough, and serves as an optical filter.
Conventionally, when the fiber grating is used in an optical circuitry, the fiber grating is connected with individual optical devices, such as a directional coupler, a Y-branch and etc., the fiber grating is connected with them via an optical fiber in most cases. Among the aforementioned optical devices, the Y-branch and the directional coupler can be fabricated from SiO.sub.2 waveguides, but the characteristic of the fiber grating is better than that of a SiO.sub.2 waveguide grating.
Moreover, the Er (erbium)-doped optical fiber is about several tens meters long and wound round a reel in most cases. The directional coupler or the Y-branch is connected with one end of the Er-doped fiber, and a pumping light with a wavelength of 1.48 .mu.m or 0.98 .mu.m is supplied thereto. An optical signal with the wavelength in 1.55 .mu.m band is supplied from the other end of the Er-doped optical fiber. The aforementioned devices are connected with the end of the Er-doped fiber by means of arc fusion.
In the aforementioned devices, the directional coupler and the Y-branch can be fabricated from the SiO.sub.2 waveguide, but the characteristic of the Er-doped fiber is better than that of an Er-doped SiO.sub.2 waveguide.
In case that the fiber grating or the Er-doped fiber is used as an optical device, since they are connected with the directional coupler or the Y-branch by means of arc-fusion, fused portions must be reinforced after the work of arc fusion is over and become voluminous, hence miniaturization of this portion is difficult.
Moreover, the disadvantage of the optical device, such as the fiber grating, which suppresses the optical signal with a particular wavelength, is that its characteristic is changed by bending and stretching. On the other hand, in the SiO.sub.2 waveguide, functions of splitting an optical signal and demultiplexing plural optical signals have been already completed, and technology for integrally realizing these functions have been established. However, in the SiO.sub.2 waveguide, functions of amplifying and isolating the optical signal have not yet been realized.
As mentioned in the above, the devices of the fiber type and those of the SiO.sub.2 type respectively have their own advantages, and are connected with each other via an optical fiber.
Accordingly, a troublesome job concerning with dealing with the remaining optical fiber is inevitable, and miniaturization of the connected portion becomes difficult.